faceplant - that link does not seem to work.
As far as I can tell, the TXB0108 is not well suited to SD card level translation. SD cards have an internal 10K to 100K pullup on the DAT3 pin, and external pullups (in the same range) are recommended on the other data pins. The TXB0108 can handle 50K or higher pullups; lower resistances will overpower its output drivers and cause problems.
The TXS0108E (as mentioned above) is designed to handle this, and can manage 60Mbps throughput when doing 1.8V to 3.3V.
As you've said, handling the high-speed modes doesn't look easy. If that's all done in hardware then it's probably not possible (because there's no way to tell when the hardware has switched modes, and without that you can't change the voltage). I'm just going to live with the lower-speed modes for now; they'll probably still be faster than most SD cards can write, and 99% of the time I'll be using the internal flash anyway.